hwmon: Only call vid_which_vrm() when needed

Some hardware monitoring drivers create the VID/VRM interface files
conditionally depending on the chip model or configuration. We should
only call vid_which_vrm() when we are actually going to create the
files. Not only it is more logical and efficient that way, but it also
prevents printing unnecessary warnings such as the one reported here:
http://lists.lm-sensors.org/pipermail/lm-sensors/2007-February/018954.html



Signed-off-by: Jean Delvare <khali@linux-fr.org>